Three current sensors are normally needed to measure the currents in each phase of a three phase motor. Many control methods and systems for commutating the switches which selectively supply power to each of the three phases of a motor require three signals, one from each of the three current sensors in each phase, for operation. Some control methods and systems employ the signal from a current sensor connected to a bus supplying current to the three phases. The cost of the sensors and the related conditioning circuitry associated with each sensor adds cost to the motor drive. The sensors also take up additional space thereby making the overall size of the drive circuitry of the motor larger.
Single current sensors are used in six step drives for three phase motors when individual phase current information is not needed. If individual phase current information is needed, more complex control methods and systems are employed.
As another example, U.S. Pat. No. 6,642,690 discloses modifying PWM signals in one cycle to allow for measurement of the current in a particular phase. After measurement, the PWM signals are again modified to correct for any distortion caused by the first modification during which the current was measured. As another example, U.S. Pat. No. 6,735,537 suggests a procedure for measuring the current in each phase of a three phase motor via a single sensor by shifting the drive signals applied to windings for individual phases only for specific cases of signal overlap to allow separating individual phase currents. As another example, U.S. Pat. No. 6,914,409 relates to a current detection method which varies the offset of phase drive signals to allow separating each phase current from the combined current signal. The offset varies depending on the particular duty cycle of the PWM phase signals. In summary, such current sensing tends to dynamically shift the drive signals in real time.